Campbell Scientific CR510 Basic Datalogger - Manual (Page 159)

SECTION 13. CR510 MEASUREMENTS
13-15
Calculating the actual resistance of a sensor
which is one of the legs of a resistive bridge
usually requires the use of one or two Processing
Instructions in addition to the bridge
measurement instruction. Instruction 59 takes a
value, X, in a specified input location and
computes the value MX/(1-X), where M is the multiplier and stores the result in the original
location. Instruction 42 computes the reciprocal
of a value in an input location. Table 13.4-2 lists
the instructions used to compute the resistance of
any single resistor shown in the diagrams in
Figure 13.4-1, provided the values of the other
resistors in the bridge circuit are known.
TABLE 13.4-2. Calculating Resistance Values from Bridge Measurement
Instr. Result Instr.# Multiplier; Offset
4 X = V
x
(R
s
/(R
s
+R
f
))
X/V
x
4 1/V
x
0
R
s
= R
f
________ 59 R
f
1-X/V
x
1 4 1/V
x
0
R
f
= ____________________ 59 1/R
s
((X/V
x
)/(1-X/V
x
))/R
s
42
5 X = R
s
/(R
s
+R
f
)
X 5 1 0
R
s
= R
f
_____ 59 R
f
1-X
1 5 1 0
R
f
= _____________ 59 1/R
s
(X/(1-X))/R
s
42
6,8,9* X = 1000 [R
3
/(R
3
+R
4
)-R
2
/(R
1
+R
2
)] *used for full bridge
6 or 9 -0.001;
1 8 1/V
x
R
1
= _______________ 6,8, or 9 R
3
/(R
3
+R
4
)
(X
1
/(1-X
1
))/R
2
59 1/R
2
42
where X
1
= -X/1000 + R
3
/(R
3
+R
4
)
R
2
= R
1
(X
2
/(1-X
2
)) 6 or 9 -0.001 R
3
/(R
3
+R
4
)
where X
2
= X
1
59 R
1
R
3
= R
4
(X
3
/(1-X
3
)) 6 or 9 0.001 R
2
/(R
1
+R
2
)
where X
3
= X/1000 + R
2
/(R
1
+R
2
) 59 R
4
1 6 or 9 0.001 R
2
/(R
1
+R
2
)
R
4
= _______________
(X
4
/(1-X
4
))/R
3
59 1/R
3
42
where X
4
= X
3
7&9* X = R
s
/R
f
* used as half bridge
R
s
= R
f
X 7 or 9 R
f
0
R
f
= R
s
/X 7 or 9 1/R
s
0
42